Any “interested party” can file a lawsuit to challenge the validity of a Will. An "interested party," called a "contestant," is someone who would become entitled to receive all or a potion of the estate if the Will were found to be invalid. The contestant can seek to invalidate the entire Will or just some of its provisions. California law requires that you cite a specific legal reason to contest the Will; it is not enough to challenge the Will because you do not like the gifts in the Will. To win a will contest, the contestant must show that the Will was the result of improper execution, lack of capacity, undue influence, or fraud.

As part of the probate process, the executor of the Will is required to notify all of the beneficiaries named in the Will, as well as the decedent's heirs, of the probate proceedings, by sending them written notice via US Mail. The superior court will hold a hearing on the petition for admission of the Will to probate at the time specified in the notice, which may be as little as 15 days after the date the notice is mailed to you. Once you receive notice of a Will being offered for probate, you must determine if you have a basis to challenge the Will and file a contest promptly. There are strict time limitations for filing a contest; failing to file the contest within the limitations may cause your contest to be dismissed. These limits can be complicated to determine, so it is best for you to contact an experienced probate lawyer to determine if you have a basis for a timely challenge to a Will.

Undue influence means taking advantage of another’s weaker state of mind. A challenge of undue influence means the decedent made the Will because another person improperly persuaded him or her to do so. But for the improper action of this third party, the person making the Will would not have signed it.

Under California law, a typed Will must be signed by the person making the Will (called the "testator") and two witnesses. Although the witnesses do not need to know the contents of the Will, the witnesses must be in the presence of the testator when the Will is signed or acknowledged. If a Will is not properly executed, it will not standup to a contest.

If a Will is completely in the testator's handwriting (called a holographic Will), it must be in the decedent's own handwriting (i.e., the decedent cannot dictate the will to another person). The Will must state that the decedent intended for the document to be a Will, and clearly state who is to receive the decedent's property. The document must be signed by the decedent.

Contrary to popular misconception, neither a typewritten Will nor a holographic Will needs to be notarized to be valid in California.

For a will to be valid, a testator must have possessed “testamentary capacity” when the Will was made. To challenge the Will on this basis requires proving that the testator did not understand the nature of his or her property or understand that he or she was signing a document to direct distribution of such property after his or her death. Expert witnesses and a medical background analysis of the testator's mental health maybe required to determine if the testator lacked mental capacity.

A Will can be challenged because of fraud if the Will is a fake or altered in some manner. Fraudulent alterations can be adding or removing something. Mistakes in the Will can also be a target for litigation, even when fraud is not alleged.